Critical decline ... some ecologists are worried about a long-term fall in kangaroo numbers.

The NSW Scientific Committee is examining a submission that challenges the science behind kangaroo census data. The most recent studies for the Office of Environment and Heritage put roo numbers in NSW at a fairly healthy 11 million, based on regular aerial surveys.

The ecologist who wrote the submission, Ray Mjadwesch, spent years comparing kangaroo population data, gathered at a national level for commercial harvesting of the animals, with the situation on the ground.

His findings, which have the support of some animal welfare groups, suggest critical declines in roo numbers are not being picked up by aerial surveys.

''When I started to look at the situation on the ground, I was quite shocked,'' Mr Mjadwesch said. ''There are huge areas where, according to the data, there should have been thousands of kangaroos, but the landscape was just empty.''

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Mr Mjadwesch said roo numbers are continuing a long-term decline of 90 to 98 per cent in many districts since European settlement, under pressure from development, land clearing and commercial shooting. The submission says steep declines had continued in the last few years - at rates sufficient to trigger an endangered species listing for some other animals, like koalas.

A spokeswoman for the NSW Scientific Committee confirmed the submission is being assessed, but would not comment further.

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The head of the state's kangaroo management program, Nicole Payne, said the methods used to measure roo numbers were robust.

''I think the difference here is to do with the scale of the surveys,'' Ms Payne said. ''If you are on foot in a relatively small area, you might see no kangaroos. When we're flying aerial surveys, people are counting the kangaroos that they see, and there's also the 'correction factor' for the ones you don't see. It has been proven to be a reliable method.''

While roo numbers fluctuate due to drought, the long-term trend pointed to healthy populations, she said.